The standard and always popular "Property Of" came to mind, as all you would really need to do is change the lettering. You could even include your sponsoring club name rather than use "Athletic Department", or use anything you like for that matter.
So although this has been done since the dead sea was just getting sick, I've added this design to My Dart Shirts available on tees, sweatshirts, pullovers, boxers, a thong (ya really) and youth dart shirts. Sending in pics of players wearing the thong is recommended. Wait, what!?